Background. Prior population-based twin studies from two Anglophonic countries with relatively high rates of drug use – the USA and Australia – suggest that genetic factors contribute substantially to individual differences in the use, abuse and dependence of illicit psychoactive substances. Would these results replicate in Norway, a Nordic country with a low prevalence of illicit drug use?

Method. Lifetime use, abuse and dependence of five illicit drug categories (cannabis, stimulants, opiates, cocaine and psychedelics) were assessed at personal interview in 1386 complete young adult twin pairs ascertained from the Norwegian Institute of Public Health Twin Panel. Twin model fitting was performed using the Mx statistical package on three phenotypes: any lifetime use, endorsement of at least one DSM-IV symptom of abuse or dependence, and meeting DSM-IV criteria for abuse or dependence.

Results. Significant lifetime use of illicit substances (defined as use 10 or more times) was reported by only 6·4% of the sample. Meaningful analyses were possible for use of any substance and each of the five substances individually, but for symptoms or a diagnosis of abuse/dependence meaningful analyses were possible only for any substance and cannabis. Full twin models uniformly found twin resemblance to be due largely or entirely to genetic factors. Best-fit models for all analyses included only genetic and individual-specific environmental effects with heritability estimates ranging from 58% to 81%.

Conclusion. In accord with prior results from the USA and Australia, genetic factors appear to play an important role in the etiology of use and abuse/dependence of illicit drugs in Norway.